The optical head device and the optical information recording/reproducing device for performing at least either recording or reproducing on the optical recording medium include a function of detecting a focus error signal and a track error signal. The Foucault's method (or a double knife-edge method), an astigmatic method, a spot size method, and the like are known as a method of detecting the focus error signal. Optical recording media in a write-once type and a rewritable type include a groove formed thereon for tracking. When a light focusing spot formed on an optical recording medium by an optical head device transects the groove, noise is generated in a focus error signal.
The noise above is smaller in the Foucault's method than the astigmatic method and the spot size method. This character becomes remarkable in the rewritable optical recording media (DVD-RAM, HD DVD-RW, etc.) with a land/groove recording/reproducing system in which recording or reproducing are performed for a LAND of a concave region in the groove and a GROOVE of a convex region in the groove. Accordingly, the Foucault's method is generally used to detect a focus error signal for those optical recording media.
On the other hand, in order to detect a track error signal, a phase-contrast method is generally used for optical recording media of a playback-only type (DVD-ROM, HD DVD-ROM, etc.), and a push-pull method is used for the write-once type (DVD-R, HD DVD-R, etc.) and the rewritable type (DVD-RAM, HD DVD-RW, etc.).
Therefore, in order to be applicable for all types of the optical recording media, such as the playback-only type, the write-once type and the rewritable type, an optical head device and an optical information recording/reproducing device are required to include a function of detecting a focus error signal by the Foucault's method, and detecting a track error signal by the phase-contrast method and the push-pull method. In order to downsize the optical head device, reflected light from an optical recording medium need to be received by a same photodetector to detect those signals. Patent Document 1 discloses an optical head device which receives reflected light from an optical information medium at the same photodetector in order to detect a focus error signal by the Foucault's method and a track error signal by the phase-contrast method and the push-pull method.
FIG. 16 shows the optical head device recited in Patent Document 1. Emitting light from a semiconductor laser 1 is parallelized by a collimator lens 2, and the light injects into a polarization beam splitter 3 as P polarization to be transmitted by almost 100%, and then it is transmitted through a quarter wavelength plate 4 to be converted from linear polarization into circular polarization, and the light is collected on a disc 6 by a objective lens 5. Reflected light from the disc 6 is transmitted through the objective lens 5 inversely, and is transmitted through the quarter wavelength plate 4 to be converted from the circular polarization into linear polarization having an orthogonal direction to the linear polarization of an incoming way, and injects into the polarization beam splitter 3 as S polarization to be reflected by almost 100%, and then is diffracted by a diffractive optical element 7e, and is transmitted through a convex lens 8, and is received by a photodetector 9c. 
FIG. 17 shows a plan view of the diffractive optical element 7e. The diffractive optical element 7e has a diffraction grating formed therein which is divided into four, regions 12i-12l, by a line passing through an optical axis of an incident light and parallel to a radical direction of the disc 6, and a line passing through the optical axis of the incident light and parallel to a tangential direction of the disc 6. Each direction of the diffraction grating is parallel to the tangential direction of the disc 6, and each pattern in the diffraction grating is linear at a regular pitch. The pitch of the diffraction grating narrows from the regions 12i, 12j, 12k, 12l in order. In this regard, a circle 5a illustrated with dotted lines in the drawing corresponds to an effective diameter of the objective lens 5. A light beam injects into the regions 12i, 12j, 12k, 12l is diffracted by about 10% to be negative first order diffracted light, and is also diffracted by about 71% to be positive first order diffracted light.
FIG. 18 shows a pattern with light receiving sections in the photodetector 9c and an arrangement of optical spots on the photodetector 9c. Optical spots 31a and 31b correspond to negative first order diffracted light from the regions 12i and 12j of the diffractive optical element 7e respectively, and are received by light receiving sections 30a and 30b into which a light receiving section is divided by a dividing line parallel to a radial direction of the disc 6. Optical spots 31c and 31d correspond to negative first order diffracted light from the regions 12k and 12l of the diffractive optical element 7e respectively, and are received by light receiving sections 30c and 30d into which a light receiving section is divided by a dividing line parallel to the radial direction of the disc 6. An optical spot 31e corresponds to positive first order diffracted light from the region 12i of the diffractive optical element 7e, and is received by a single light receiving section 30e. An optical spot 31f corresponds to positive first order diffracted light from the region 12j of the diffractive optical element 7e, and is received by a single light receiving section 30f. An optical spot 31g corresponds to positive first order diffracted light from the region 12k of the diffractive optical element 7e, and is received by a single light receiving section 30g. An optical spot 31h corresponds to positive first order diffracted light from the region 12l of the diffractive optical element 7e, and is received by a single light receiving section 30h. 
Outputs from the light receiving sections 30a to 30h are represented by V30a to V30h respectively. Then, a focus error signal according to the Foucault's method can be obtained from calculation of (V30a+V30d)−(V30b+V30c). A track error signal according to the phase-contrast method can be obtained from a phase difference between (V30e+V30h) and (V30f+V30g). A track error signal according to the push-pull method can be obtained from calculation of (V30e+V30g)-(V30f+V30h). Further, an RF signal recorded on the disc 6 can be obtained from calculation of (V30e+V30f+V30g+V30h). In the optical head device disclosed in patent document 1, the configurations of the photodetector and the computation circuit are simple since only four light receiving sections for detecting the focus error signal and four light receiving sections for detecting the track error signal are necessary.
Patent document 1: Japanese Laid-Open Patent Publication No. 2004-139728